The prior art which is most closely related to that of the present invention is that covered by U.S. Pat. No. 5,328,309 (Bishop et al), U.S. Pat. No. 5,292,214 (Bishop et al), U.S. Pat. No. 5,390,408 (Bishop et al) and U.S. Pat. No. 5,131,430 (Roeske). The first three of these prior art references relate to "slotting machines" which machine the slots in the bore of power steering valve sleeves. The design of these machines calls for each of the slots to be scooped out of the bore of a turned blank by a finger-like cutting tool mounted in a cutting spindle which angularly reciprocates about an axis in a series of progressively deeper cutting and return strokes so forming a closed chamber, or hydraulic port, which is arcuate in longitudinal section. The sleeve is held in a work holding collet, in turn mounted in a work holding spindle, having a rotational axis perpendicular to and offset with respect to the cutting spindle axis. By accurately indexing the work holding spindle upon completion of each slot, the required number of slots are precisely machined in the sleeve, usually 4, 6 or 8 slots for most automotive applications. The slots formed by such machines are arcuate in shape and typically blind-ended.
U.S. Pat. No. 5,131,430 (Roeske) relates to sleeves having a plurality of slots, where some of the slots extend to the axial extremity of the sleeve and are formed to provide hydraulic fluid return paths to the return cavity at one axial extent of the sleeve, and hence to the return port of the power steering valve. In these sleeves such "return" slots are necessarily axially longer in extent and axially displaced with respect to the "inlet" slots, the latter being fully-blind-ended. The slots can be manufactured in a number of ways, the first of which disclosed in the above referenced U.S. Pat. No. 5,292,214 (Bishop et al), is by journalling the workholding spindle of a slotting machine for both sliding and rotation. By varying the axial position of the work holding spindle between two predetermined positions and varying the maximum infeed depth of the cutting tool between two predetermined depths, and by mechanically synchronising these two motions with the rotational index position of the work holding spindle, a sleeve incorporating return slots of the type described can be manufactured. This is achieved by firstly say, machining an inlet slot, then indexing and axially shifting the sleeve such that an axially displaced return slot can be machined. The sleeve is then indexed and axially shifted once again, so that a further inlet slot can be machined. The indexing and axially shifting takes place after each machining operation until the full array of slots have been machined.
Another way of producing the slotted sleeves as disclosed in U.S. Pat. No. 5,131,430, is to double process the sleeves through two prior art slotting machines. For an eight slot sleeve for example, a first slotting machine could slot the four return slots and a second machine the four axially shorter inlet slots.
In many conventional rotary valves a leakage cavity exists at one axial extremity of the sleeve in the housing in order to accommodate leakage oil which inevitably leaks out the input shaft/sleeve journal area and the sleeve/housing interface. This leakage cavity must be communicated to the return port to avoid pressure build up. In many rotary valves this communication is via a breather hole in the input-shaft. Such a "breather hole" arrangement is shown but not referenced on the input shaft shown in FIGS. 1 and 2 of U.S. Pat. No. 5,233,906 (Bishop et al).
In some instances sleeves are arranged such that the return slots are deep and run the axial length of the sleeve to provide communication between the leakage and return cavities. One example of a rotary valve with such a return slot configuration is disclosed in U.S. Pat. No. 4,335,749 (Walter). In this arrangement the three return slots which run the axial length of the sleeve are broached into the sleeve, whilst the remaining slots are slotted utilising a prior art slotting machine. A disadvantage of a broaching operation is that it results in less accurate machining tolerances for metering edges on the broached slots. It is well understood in the art of power steering that accurately positioned metering edges on the slots produce better control of hydraulic and noise characteristics of the valve, and that less accurate machining tolerances such as produced by broaching are not desirable. A further disadvantage of the sleeve of U.S. Pat. No. 4,335,749 is that it must be slotted in two separate machine operations.
It is therefore a problem with the prior art that broached return slots machined the axial length of the sleeve cannot be produced with the same metering edge accuracy and efficiency as can slots machined by slotting machines.